US 2006/0197990 A1 discloses a superposition of two tally images, thus revealing a hidden image. The hidden image cannot be reconstructed from a single tally image.
US 2013/0181435 A1, which is incorporated by reference in its entirety, inter alia discloses a model for computing surface coverage in order to obtain a desired color.
WO 2009/056355 A1 discloses a security document with several substrate layers. An information is separated into at least two print excerpts which are printed on at least two different substrate layer surfaces such that the printed print excerpts are laid over each other.